It has been known for a long time to measure potentials on the surface of an area of the earth and that the potentials give information on the strength and course of subterranean disturbances, especially dynamic changes. For example, it is known, for this purpose, to pass high currents into the bedrock and to successively measure potentials in the region around the current introduction point. It is furthermore known to determine self-potentials with respect to a base probe.
Determination of the potentials can yield good results particularly during dynamic bedrock changes such as in artificial fracture production. Breaking the rock by means of hydraulic pressure results in seismic or seismic-acoustic events and electrical conductivity changes. In particular, filtration potentials and potentials on the basis of a geochemical change in bore flushing liquid are produced as the result of injecting liquid or steam into the rock formation at high pressure and in a short time. In conjunction with this, an electrolyte is compressed by the rock capillaries which act like a diaphragm. The electrochemical double layer which originally exists in solid/fluid form on the boundary surface is in consequence decomposed into a stationary part and a non-stationary diffusive part. This results in a potential difference as a consequence of the electrical charges between the ends of the capillaries.
However, the potentials to be measured at the surface are very small.
Naturally and artificially produced disturbance signals result from telluric currents which are caused, for example, by industrial or railway systems and whose amplitudes are above the order of magnitude of the potential differences to be measured. Known methods can thus be used only to a limited extent for detecting changes in conditions in the bedrock.
EP 477,167 discloses a method for monitoring an area wherein the potential is measured by means of probes at a plurality of measurement points in the area and is converted into electrical signals, the signals being interrogated and processed at specific time intervals by a central computer unit. The measurement points are in this case arranged at predetermined intervals along a closed line which encloses the area. In addition, a number of measurement points are provided underneath the area. This known method can be used, in particular, for detecting waste dumps in terms of potential. This method is based on measuring long-term changes in the condition of a dump. It relates to measuring potential differences from one measurement to the next measurement without being able to make any statement about the absolute condition of the area.
German patent document 962,105 discloses a method for locating and prospecting for usable deposits in the ground with the aid of self-potential measurements, a current conductor having a high controllable resistance being connected in parallel with the input of the potential test set after measuring the self potential of the ground to be investigated. That resistance value is determined at which the potential of the ground, when the resistance is switched on, falls to a fraction of the potential value measured without current consumption.
German 392,189 also discloses a method for determining the earth resistance in conjunction with potential measurements.
German 3,529,189 discloses a method for determining limits of subterranean deposits which are filled with hydrocarbons wherein measurement probes are arranged in a grid and self-potential curves are detected.